Tutorial
Error Handling

Error Handling

In the previous chapter, you encountered an error while executing a Mutation.postCommentOnLink operation. In this chapter, you will learn the origin of this error and learn how to properly handle it.

Recap of the Encountered Error

This is the resolver implementation that caused the issue when using a linkId that has no counter-part within the database.

src/schema.ts
const resolvers = {
  // ... other resolver maps ...
  Mutation: {
    // ... other Mutation object type field resolver functions ...
    async postCommentOnLink(
      parent: unknown,
      args: { linkId: string; body: string },
      context: GraphQLContext
    ) {
      const newComment = await context.prisma.comment.create({
        data: {
          linkId: parseInt(args.linkId)
          body: args.body,
        }
      })
 
      return newComment
    }
  }
}

Execute the following GraphQL operation on GraphiQL:

mutation postCommentOnLink {
  postCommentOnLink(linkId: "99999999999", body: "This is my second comment!") {
    id
    body
  }
}

Again, this should yield the following error response:

{
  "errors": [
    {
      "message": "Unexpected error.",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": ["postCommentOnLink"],
      "extensions": {
        "originalError": {
          "message": "\nInvalid `context.prisma.comment.create()` invocation in\nhackernews/src/schema.ts:69:52\n\n   66   args: { linkId: string; body: string },\n   67   context: GraphQLContext,\n   68 ) => {\n→  69   const comment = await context.prisma.comment.create(\n  Foreign key constraint failed on the field: `foreign key`",
          "stack": "Error: \nInvalid `context.prisma.comment.create()` invocation in\nhackernews/src/schema.ts:69:52\n\n   66   args: { linkId: string; body: string },\n   67   context: GraphQLContext,\n   68 ) => {\n→  69   const comment = await context.prisma.comment.create(\n  Foreign key constraint failed on the field: `foreign key`\n    at cb (hackernews/node_modules/@prisma/client/runtime/index.js:38703:17)\n    at PrismaClient._request (hackernews/node_modules/@prisma/client/runtime/index.js:40859:18)"
        }
      }
    }
  ],
  "data": null
}

As you can see the error includes an extensions originalError field. It includes a full stacktrace of the original error for finding the cause easily.

Yoga Error Masking

In a production environment, a stacktrace that leaks to the outside world are a potential security threat as that information could be misused by malicious actors.

The extensions originalError field is only present within an error if the server has been started with the NODE_ENV environment variable being set to development.

As you might remember this is only the case when you are starting the server using npm run dev.

Start the GraphQL server using the npm run start script.

npm run start

Then execute the same operation again.

mutation postCommentOnLink {
  postCommentOnLink(linkId: "99999999999", body: "This is my second comment!") {
    id
    body
  }
}

Now you can see that there is no originalError within the errors array of the first object.

{
  "errors": [
    {
      "message": "Unexpected error.",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": ["postCommentOnLink"]
    }
  ],
  "data": null
}

This is great, but now we never know why our request failed 🤔 and the error message Unexpected error. is also not helpful to anyone not aware of the exact resolver function implementation.

Exposing Safe Error Messages

Let’s change the implementation to expose the much more helpful message Cannot post comment on non-existing link with id 'X'. instead of Unexpected error..

For doing so you will use the GraphQLError class that is exported from the graphql package.

Add the following catch handler for mapping a foreign key error into a GraphQLError:

src/schema.ts
// ... other imports ...
import { GraphQLError } from 'graphql'
import { Prisma } from '@prisma/client'
 
// ... other imports ...
 
const resolvers = {
  // ... other resolver maps ...
  Mutation: {
    // ... other Mutation object type field resolver functions ...
    async postCommentOnLink(
      parent: unknown,
      args: { linkId: string; body: string },
      context: GraphQLContext
    ) {
      const newComment = await context.prisma.comment
        .create({
          data: {
            body: args.body,
            linkId: parseInt(args.linkId)
          }
        })
        .catch((err: unknown) => {
          if (err instanceof Prisma.PrismaClientKnownRequestError && err.code === 'P2003') {
            return Promise.reject(
              new GraphQLError(`Cannot post comment on non-existing link with id '${args.linkId}'.`)
            )
          }
          return Promise.reject(err)
        })
 
      return newComment
    }
  }
}

The error code P2003 indicates a foreign key constraint error. You can learn more about it in the Prisma documentation. You use that code for identifying this edge case of a missing link and then throw a GraphQLError with a useful error message instead.

Restart the server using npm run dev and again execute the mutation operation using GraphiQL.

mutation postCommentOnLink {
  postCommentOnLink(linkId: "99999999999", body: "This is my second comment!") {
    id
    body
  }
}

You will receive a response with the Cannot post comment on non-existing link with id '99999999999'. message:

{
  "errors": [
    {
      "message": "Cannot post comment on non-existing link with id '99999999999'.",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": ["postCommentOnLink"]
    }
  ],
  "data": null
}

As you might have noticed, GraphQL Yoga will exclude wrapped errors thrown within the resolvers from masking the error masking.

That means every time you want to expose an error to the outside world you should throw such an error.

At the same time, any other unexpected error will automatically be masked from the outside world, bringing you sensible and safe defaults for a GraphQL Yoga production deployment!

Field Argument Sanitizing

There is one more issue within The Mutation.postCommentOnLink field resolver.

So far you only executed the mutation operation while using a string encoded integer value for the linkId argument.

Execute that operation again using this non-integer value:

mutation postCommentOnLink {
  postCommentOnLink(linkId: "11a", body: "This is my second comment!") {
    id
    body
  }
}

All seems good, right?

{
  "errors": [
    {
      "message": "Cannot post comment on non-existing link with id '11a'.",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": ["postCommentOnLink"]
    }
  ],
  "data": null
}

Let’s also try another non-integer string value.

Execute that operation again using this non-integer value:

mutation {
  postCommentOnLink(linkId: "uuuuuu", body: "This is my second comment!") {
    id
    body
  }
}

What the heck is happening now?

{
  "errors": [
    {
      "message": "Unexpected error.",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": ["postCommentOnLink"],
      "extensions": {
        "originalError": {
          "message": "\nInvalid `.create()` invocation in\nhackernews/src/schema.ts:93:10\n\n   90   context: GraphQLContext,\n   91 ) => {\n   92   const comment = await context.prisma.comment\n→  93     .create({\n            data: {\n              body: 'This is my second comment!',\n              linkId: NaN\n              ~~~~~~\n            }\n          })\n\nUnknown arg `linkId` in data.linkId for type CommentCreateInput. Did you mean `link`? Available args:\ntype CommentCreateInput {\n  createdAt?: DateTime\n  body: String\n  link?: LinkCreateNestedOneWithoutCommentsInput\n}\n\n",
          "stack": "Error: \nInvalid `.create()` invocation in\nhackernews/src/schema.ts:93:10\n\n   90   context: GraphQLContext,\n   91 ) => {\n   92   const comment = await context.prisma.comment\n→  93     .create({\n            data: {\n              body: 'This is my second comment!',\n              linkId: NaN\n              ~~~~~~\n            }\n          })\n\nUnknown arg `linkId` in data.linkId for type CommentCreateInput. Did you mean `link`? Available args:\ntype CommentCreateInput {\n  createdAt?: DateTime\n  body: String\n  link?: LinkCreateNestedOneWithoutCommentsInput\n}\n\n\n    at Object.validate (hackernews/node_modules/@prisma/client/runtime/index.js:34786:20)\n    at PrismaClient._executeRequest (hackernews/node_modules/@prisma/client/runtime/index.js:40911:17)\n    at consumer (hackernews/node_modules/@prisma/client/runtime/index.js:40856:23)\n    at hackernews/node_modules/@prisma/client/runtime/index.js:40860:76\n    at runInChildSpan (hackernews/node_modules/@prisma/client/runtime/index.js:39945:12)\n    at hackernews/node_modules/@prisma/client/runtime/index.js:40860:20\n    at AsyncResource.runInAsyncScope (async_hooks.js:197:9)\n    at PrismaClient._request (hackernews/node_modules/@prisma/client/runtime/index.js:40859:86)\n    at hackernews/node_modules/@prisma/client/runtime/index.js:40190:25\n    at _callback (hackernews/node_modules/@prisma/client/runtime/index.js:39960:52)"
        }
      }
    }
  ],
  "data": null
}

You just encountered another unexpected error that got masked.

Let’s analyze the error message:

Invalid `.create()` invocation in hackernews/src/schema.ts:93:10

Unknown arg `linkId` in data.linkId for type CommentCreateInput.

linkId: NaN

These are the three parts within the error message that should make it click.

It seems like the linkId passed to the prisma create function is NaN, which is a special value for “not a number” in JavaScript. The underlying database, however, expects an integer value. The error is raised and thrown, as you did not add logic for handling this edge case, yet.

But why did the previous value "11a", not result in such an error?

To clarify that question we need to have a small excursion on how the parseInt function works.

There are multiple mathematical numeral systems. The implementation tries to be a bit smart and fault-tolerant. So, let’s build a quick custom function, parseIntSafe, that first validates the string contents using a regex.

Add the following code to the src/schema.ts file.

src/schema.ts
// ... other code ...
import { GraphQLError } from 'graphql'
import { Prisma } from '@prisma/client'
 
const parseIntSafe = (value: string): number | null => {
  if (/^(\d+)$/.test(value)) {
    return parseInt(value, 10)
  }
  return null
}
 
const resolvers = {
  // ... other resolver maps ...
  Mutation: {
    // ... other field resolver functions
    async postCommentOnLink(
      parent: unknown,
      args: { linkId: string; body: string },
      context: GraphQLContext
    ) {
      const linkId = parseIntSafe(args.linkId)
      if (linkId === null) {
        return Promise.reject(
          new GraphQLError(`Cannot post comment on non-existing link with id '${args.linkId}'.`)
        )
      }
 
      const newComment = await context.prisma.comment
        .create({
          data: {
            linkId,
            body: args.body
          }
        })
        .catch((err: unknown) => {
          if (err instanceof Prisma.PrismaClientKnownRequestError) {
            if (err.code === 'P2003') {
              return Promise.reject(
                new GraphQLError(
                  `Cannot post comment on non-existing link with id '${args.linkId}'.`
                )
              )
            }
          }
          return Promise.reject(err)
        })
      return newComment
    }
  }
}

The regex /^(\d+)$/ simply verifies that every character within the value is a digit. In case the value includes a non-digit character, you simply return null and then reject the resolver with a GraphQLYogaError error.

Returning null instead of returning NaN is the better choice here, as it allows nice TypeScript checks (linkId === null).

Execute that operation again using this non-integer value:

mutation postCommentOnLink {
  postCommentOnLink(linkId: "uuuuuu", body: "This is my second comment!") {
    id
    body
  }
}

All is good now. 🎉

{
  "errors": [
    {
      "message": "Cannot post comment on non-existing link with id 'uuuuuu'.",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": ["postCommentOnLink"]
    }
  ],
  "data": null
}

Remember, when implementing your GraphQL resolver functions, the input arguments should always be sanitized and validated!

Optional Exercise

As an optional exercise for interiorizing the knowledge, you can now also implement validation of the body argument of Mutation.postCommentOnLink and argument sanitization for the Mutation.postLink field. You wouldn’t want anyone to post an empty comment or invalid link, right?

Graph RelationsFiltering and Pagination